Vapor phase corrosion inhibition



United States Patent VAPOR CORROSION INHIBITION Application February 24,1958 Serial No. 716,847

8 Claims. (Cl. 117-106) No Drawing.

"This invention relates to inhibiting the corrosion of v metal surfacesin enclosed spaces, and specifically to effecting such inhibition bybringing at least two mutually reactive substances into contact withsuch surface and reacting them together thereon to form a layer orcoating of the inhibiting substance, at least one and preferably all ofsaid reactive substances being in the vapor phase. The presentapplication is a continuation-in-part of my .copending applicationSerial No. 483,837 filed January .24, ,1955. 1 1 There are many pieces'of mechanical equipment containing intricate metal parts which are usedonly intermittently. Engines, including gasoline, diesel, jet etc., farmand marine equipment, machine tools, containers, bins, etc., areillustrative of items which have inter- .mittent periods of use. It isdesirable to have a rapid and easy method for protecting metal surfacesforming a part of these items. Since many of them contain parts whichbecause of their location or intricacy are inaccessible to the commonmethods of applying an anti-corro'sion coating (such as painting,spraying or dipping) it isnecessary to find a way of depositing aprotective coating which may be carried into and around these intricate-or inaccessible parts by means of a gas or easily introduced liquid.

i There are a number of methods known for protecting metal surfacesagainst moisture and corrosive vapors, but a fuels, which remain in anengine or jet motor after shut 1 downs. These corrosion inhibitors arealso very'eifective against atmospheres of excessively high humidities.

i :.'The' idea of vapor phase inhibition of corrosion is not new sincevarious compounds capable ofbeing vaporized have been deposited on metalparts to'prevent corrosion. 1.

In the usual process, the vaporized inhibiting compound isv carried bymeans of air to contact the metal surfaces -to'be'protected. Contrary tosome contentions, it appears clear that the inhibitor is not effectivewhile it is in the vaporphase but it must be deposited on the metal andg; subsequently dissolved in any drops of moisture, formed on the metalsurface, so as to render said drops noncorrosive' and harmless to themetal. The inhibitor which is actually active is that dissolved in thedeposited moisture. This mechanism partially defeats the purpose of thecommonly used vapor phase inhibitors since they may well revaporize fromthe metal surfaces before they have an opportunity to act in the mannerdescribed. In order to be completely effective under conditions ofexcessive humidity the vapor phase inhibitor should be hydrobustionencountered in engines and jet motors.

phobic in nature and preferably should be capable; of forming awater-repelling coating.

The rate at which the inhibitor will evaporate depends upon thetemperature involved, the vapor pressure of the inhibitor, and the rateat which air passes over the metal surface of the object. If betterprotection is sought at higher temperatures it is necessary to selectvapor phase inhibitors which vaporize at higher temperatures and whichare, accordingly, more difficult to apply if formed before application.

In the prior art many vapor phase inhibitors have been disclosed buttheir deposition on metal surfaces has simply been left to chance and nospecial means were used to cause them to deposit. This is particularlytrue since vapor phase corrosion inhibitors are commonly employed inequipment where other means of application such as brushing, spraying,etc., cannot be used because of inaccessibility to the surfaces. Normaltemperature gradients throughout a typical piece of equipment, such as ajet engine for example, contribute to the spottiness of surfacecoatings, and hence to the lack of protection, at-

.tained. In addition, the more easily volatilized inhibitors .which hadto be used were less hydrophobic in nature.

also be one which would adhere in a continuous, thin coating to themetalsurfaces even at elevated temperatures and preferably be hydrophobic innature. I' have found that by the process of this invention suchdesirable vapor phase corrosion inhibitors can bedeposited on metalsurfaces by forming the inhibitor in situ (i.e. on such surfaces) byreacting two, or more, substances on the metal surfaces. In this mannerthe inhibitor is formed directly on the metal and by proper choice ofreactants it is possible to form an inhibitor which vaporizes at muchhigher temperaturesthan the reactants. This means that the temperaturesat which the inhibitors are applied can be lowwhile at the same time theresulting inhibitor coating maintains its eificiency at relatively hightemperatures. It also makes possible the use of the more hydrophobiccompounds as inhibitor coatings. -An object of this invention is to forma vapor phase corrosion inhibitor on enclosed metal surfaces which willadhere to the metal at elevated temperatures. Another object of thisinvention is to form on metalysurfaces a corrosion inhibitor which willbe continuous and of an even thickness. A further object of thisinvention is the formation of a vaporphase corrosion inhibitor in situat temperatures lower than those required to vaporize the resultingvapor phase corrosion inhibitor. Still another object of this inventionis to protect metal surfaces not only against corrosion due to contactwith air and waterbut also against that-dueto the products of com- Anadditional object of this invention is to provide vapor phase corrosioninhibitors which are hydrophobic in character. These and other objectswill appear in the followingdiscussion. i

The improved vapor phase corrosion inhibitors of this invention areformed by introducing two or-more"re actants which are not idthemsel-vesinhibitors into the space containing or consisting of the metal surfacesto be coated and reacting these reactants with each other'on the metalsurfaces. One and preferably all'of the reactants are in the vaporstate, to promote ready introducably strongly basic, and at leastanother of the reactants is a weak acid or an anhydride of a weak acid.

An example of a corrosion inhibitor formed in situ in accordance withthis invention is morpholine carbamate which has the structure GHQ-H2and may be formed on a metal surface by introducing morpholine vapor andcarbon dioxide or by first introducing the volatile amine carried bymeans of air, and then after the morpholine has deposited on thesurfaces, introducing the gaseous carbon dioxide to react with themorpholine. The morpholine carbamate is much less volatile than eithermorpholine or carbon dioxide. However, morpholine carbamate is somewhatvolatile and so it is frequently desirable to use less volatile aminesand acids to form even less volatile inhibitors. For example, the use ofbenzoic or caprylic acid to give morpholine benzoate or caprylateresults in a corrosion in hibitor which is only difficultly volatile andwhich is hydrophobic in character.

The amines suitable for this invention are preferably volatile attemperatures which can be conveniently handied under the circumstancesin which the corrosion inhibitor is to be used. For example, if thevapor phase corrosion inhibitor is to be used in a jet engine, it isdesirable that the amine be capable of volatilizing at or below thetemperature reached by the internal surfaces of the jet engine at thetime it is shut off. However, a difiieultly volatile amine may beintroduced without volatilizing by means of a suitable liquid such asoil which may be flushed through a mechanical system after it is turnedoff. In addition to possessing these physical properties, the amineshould preferably be capable of forming a solid compound with carbondioxide and should be sufficiently basic to give a stable solid saltwith the acid used.

The use of an amine or acid dissolved in oil is particularly useful inreciprocating engines, either gasoline or diesel, where the internalsurfaces are normally covered with oil which is pumped through theengine. In this case the special oil, containing amine or acid, can befed to the oil circulating pump just before shutting down the engine. Inother cases, it may be less con venient to apply the acid or amine inoil, but this can usually be done by oil sprays.

Since the inhibitor formed by the reaction will be much less soluble inoil than the component introduced in the oil, its formation will usuallygel the oil so that both the oil and the inhibitor will remain as a filmon the surface. This film will prevent the oil draining off as untreatedoil would do. The amine may be dispersed or dissolved in the carryingliquid. When the engine is started up the heat generated, along withfresh oil flowing in, removes the inhibitors without any further stepsbeing taken.

The amines should be stable in the vapor state and should not besubstantially corrosive to the metal surface. They should preferably bestrongly basic. Amines suitable for this invention may be aliphatic oraromatic and include, but are not limited to, morpholine, cyclo hexylamine and piperidine. Any amine of proper basicity and volatility issuitable. The stronger the acid, the less the required basic strengthrequired in the base. For example, dicyclohexyl amine is too weak to usewith carbon dioxide, but is satisfactory with caprylic acid. The threenamed above (morpholine, cycloehxyl amine and piperidine) have beenfound satisfactory with carbon dioxide and substantially all otheracids.

The weak acid, or the anhydride of a weak acid, should not besubstantially corrosive in itself. As in the case of the amine, the acidor acid anhydride should preferably be volatile under the conditions inwhich it is to be used. However, it is also possible to introduce it byusing any suitable liquid as a carrier; For example, naphthenic acidwhich is substantially non-volatile can be dissolved in oil which is fedto the lubricating pump at the end of a run. The lubricating oil willdistribute the naphthenic acid over the internal metal surfaces of theengine. The amine can then be introduced to form a ditficultly volatileamine naphthenate on the metal surfaces.

Acids suitable for this invention include, but are not limited to,carbonic, benzoic, caprylic, and naphthenic. In addition, anhydrides ofacids such as carbon dioxide may be used.

The sequence of introduction of the reactants into the enclosed spacewherein are the metals to be protected is determined by consideringseveral factors which include the relative corrosiveness of thereactants, the manner in which the reactants are to be introduced andthe relative adhering qualities of the reactants.

When all the reactants are volatile, it is desirable to introduce themin order of increasing volatility, i.e., the less volatile reactantfirst. This allows the second reactant to be introduced at a lowertemperature than that required to vaporize the first reactant, thuspermitting the first reactant to remain intact on the surface forsubsequent reaction with the second reactant. An additional reason forintroducing the less volatile component first is that the engine is hotwhen first shut down but is cooling fairly rapidly, particularly withair-cooled engines. Therefore, the less volatile material should beintroduced when the engine is hottest, that is first.

If the reactants in themselves are somewhat corrosive to the metalsurfaces to be protected, then it is desirable to put the more corrosivereactant on first, i.e. in decreasing corrosiveness with respect to thesurface to be protected. The second reactant may then be introduced inexcess to react with all of the first reactant, leaving the lesscorrosive material to be present for a short time before removal.

If one reactant is carried in a solution or dispersion, and the other ina gas, it is preferable to introduce the solution or dispersion first toprevent washing 0 of a reactant deposited on the metal surface. As acorollary to this statement, it may be noted that if two reactants havedifferent adhering qualities, the one which adheres better should beapplied first.

The quantities of reactants introduced into the system to be protectedare not critical but it is desirable, for economic reasons, to keep theratios of reactants close to the stoichiometric ratios. If the metalsurface is to be protected only under dry conditions, i.e., no liquidWater will collect on the surface, it is not necessary to form a finalcoating of the corrosion inhibitor which is more than one to a fewmolecules thick. When water, or other liquid, deposits there must besuflicient inhibitor present to dissolve in the water and render itnon-corrosive. The more water that deposits, the more inhibitor that isrequired. Also, the inhibitor must have sufficient solubility in thewater droplets to render them noncorrosive. Where there are sufficientWater deposits to cause drip-off even larger amounts of inhibitor arerequired.

The amount of liquid present, the ability of the first introducedreactant to adhere to the surfaces to be protected, the rate of reactionattained at the temperature of reaction, and the relative corrosivenessof the reactants, will be factors to be considered in setting the ratioof reactants used, as well as the total amounts of them used.

Aspointed out above, the reactant amines and'acids may be introduced asvapors or one of them may be dissolved or dispersed in a liquid. In anycase if one reactant is introduced in a solvent or a dispersion, theother reactant must be introduced as a vapor or gas. The vapor or gasmay be a pure vapor or gas of the reactant such as gaseous carbondioxide or morpholine vapor, or may be gasses carried by an inert gassuch as air or nitrogen. When the gaseous reactant or reactant's'arediluted withair or other gaseous diluent, rather than in the pure state,they should nevertheless be in effective concentration in the diluent inorder to assure reaction of the components to form the desired corrosioninhibiting product. The reactions between the amines, on the one hand,andthe acidic components, on the other, are more or less readilyreversible, and unless there is an effective concentration of thevapor-phase reactants the reaction does not go sufficiently tocompletion to provide an effective quantity of the corrosion inhibitorupon the metal surface to be protected. I have found that the gaseousreactants to be effective for the purposes of this invention, shouldeach constitute at least ten percent, and preferably-at least fifteenpercent, of the gaseous medium surrounding the metal surfaces to betreated a'nd"within the aforesaid enclosed spaces. As the concentrationsare increased above the'minima just specified, the reactions of thisinvention also proceed more rapidly and a heavier deposit will beobtained, other things being equal. If one reactant is gaseous and theother is in a liquid, the gaseous reactant will therefore be present inthe concentration indicated (i.e. and preferably of the entire gaseousmedium) and the other reactant will be present in an amount which, asindicated above, will preferably be stoichiometrically substantiallyequivalent to the gaseous reactant.

If a reactant is used as a liquid or in the form of small solidparticles and is carried into the system by means of a gas, heat forvaporization of the reactant may be supplied by the gas carrier or byradiation from the hot metal surface to be coated. Thus the reactant maybe preheated and applied to a cold surface where it con denses orsublimes or it may be applied in a relatively cold state on a hotsurface which furnishes heat to vaporize it before depositing on thesurface as the surface cools. To illustrate, morpholine, as a vapor or adispersed liquid, can be injected into the intake of a jet engine and asit comes near the hot internal surfaces of the engine it will vaporizeand deposit uniformly on the hot surfaces. As the engine cools, themorpholine will form a continuous coating on the surfaces. Subsequentintroduction of gaseous carbon dioxide will form a coating of morpholinecarbamate.

It may be desirable to introduce reactants to form more than one layerof inhibitors. Thus, it is possible to put in more than one amine andmore than one acid to get a cumulative effect. For example a strongamine and a weaker one could be introduced with an acid to get quickprotection which would not be so enduring, along with less immediateprotection which would last over a longer period of time. Or, if themetal surfaces to be protected were a combination of ferrous andnonfeirous metals a strongly hydrophobic material such as a caprylatecould be formed first to protect the nonferrous surfaces and then amorpholine salt of carbon dioxide could be deposited on top, thuseliminating the possibility of the morpholine salt attacking thenonferrous surfaces.

This invention is illustrated by, but not limited to, the followingexamples:

Example I A piece of cast iron out from an aircraft cylinder wasintroduced, at room temperature, into a fiask containing a small amountof morpholine in the bottom and in which the air was fully saturatedwith morpholine vapor a tIlSl) F. The flask and sample Were allowed toicoo lfso that the'morpholine would deposit on the metal surface. The'sample was then put into a flask containing a small piece of Dry Icesuflicient to provide at least ten percent concentration of CO in thegaseous atmosphere in the flask; the subsequently generated carbondioxide vapors reacted with the morpholine coating to form morpholinecarbamate. The coated sample showed excellent resistance to moisture.

Example 2 Example 3 Several pieces of cast iron similar to that used inExample-l were dipped in oil containing ten percent by weight ofcyclohexylamine and the excess oil solution was allowed to drain oif.Some of these oiled pieces were placed in atmospheres constituted of atleast 15% caprylic acid at about 150 ,F., others were placed inatmospheres constituted of at least 15% benzoic acid,

at about 150 F., and still others were placed in an 7 atmosphereconstituted of at least 15% carbon dioxide, at room temperature. In eachinstance, the gaseous acidic component reacted with the cyclohexylamineto form on the iron'surfaces a corrosion-inhibiting substance which wasfully protective even when the surfaces were exposed to water vaporunder conditions which would ordinarily cause rusting.

Example 4 Gaseous vapors of morpholine, cyclohexylamine and carbondioxide were injected into the intake of a diesel engine. Morpholinecarbamate formed almost immediately giving rapid and extensiveprotection. Cyclohexylamine carbamate formed at a slower rate to givefurther and more lasting protection of the internal surfaces againstmoisture and the gaseous combustion products remaining in the engine.

It will -be seen that by the use of this invention it is possible todeposit one or more coatings of a corrosion inhibitor on surfaces whichwould be otherwise unattainable or which could be reached only withdifficulty. By forming a corrosion-inhibiting compound in situ, it ispossible to protect a metal surface over longer intervals and at highertemperatures than those for which previous vapor phase corrosioninhibitors could be employed.

I claim:

1. A method for protecting an enclosed metal surface from corrosionwhich comprises reacting on said metal surface at least two reactants toform a coating of corrosion-inhibiting material in situ on said metalsurface, one of said reactants being a basic amine and the other of saidreactants being an acidic material selected from the group consisting ofweak acids and anhydrides of weak acids, at least one of said reactantsbeing in the vapor state at the time of application and being present inthe gaseous medium around said surface in a concentration of at leastten percent, said amineand said acidic material being brought separatelyinto contact with said surface for said reacting thereon.

2. A method for protecting a metal surface from corrosion whichcomprises reacting on said metal surface an amine and an acidic materialselected from the group consisting of weak acids and anhydrides of weakacids, to form a salt of the amine in situ, said amine being stronglybasic, both said amine and said acid being in the vapor state at thetime of application and each being present in the gaseous medium aroundsaid surface in a concentration of at least ten percent, said amine andsaid acidic material being brought separately into contact with saidsurface for said reacting thereon.

' 3. The method in accordance with claim 2 wherein the amine ismorpholine and the acidic material is carbon dioxide.

4. The method in accordance with claim 2 wherein the amine and theacidic material are introduced in order of increasing volatility. i 5. Amethod for protecting a metal surface from corrosion which comprisesreacting on said metal surface a strongly basic amine and a weak acid toform a salt of the amine in situ upon said surface, said weak acid beingin the vapor state at the time of application and being present in thevapor in a concentration of at least fifteen percent, said amine andsaid acid being brought separately into contact with said surface forsaid reacting thereon.

6. A method for protecting an enclosed metal surface from corrosionwhich comprises bringing into contact with said surface a firstreactant, subsequently bringing into contact with said surface a secondreactant which is reactive with said first reactant, reacting on saidmetal surface said two reactants tov form a coating ofcorrosion-inhibiting material in situ on said metal surface, one of saidreactants being a basic amine and the other of said reactants being amember of the group consisting 'of weak acids and anhyrides of weakacids, at least one of said reactants being in the vapor state at thetime of application and being present in the gaseous medium around saidsurface in a concentration of at least ten percent.

7. The method in accordance with claim 6 wherein said first reactant isintroduced into contact with said surface inaliquid'vehicle. V

8. The method in accordance with claim 7 wherein said vehicle islubricating oil.

References Cited in the file of this patent UNITED STATES PATENTSMatuszak Feb. 26, 1952 OTHER REFERENCES Baker: Volatile Rust Inhibitors,NRL Report 4319; March 10, 1954, Naval Research Laboratory, Wash, D.C.(Copy in Scientific Library.)

sums.

1. A METHOD FOR PROTECTING AN ENCLOSED METAL SURFACE FROM CORROSIONWHICH COMPRISES REACTING ON SAID METAL SURFACE AT LEAST TWO REACTANTS TOFORM A COATING OF CORROSION-INHIBITING MATERIAL IN SITU ON SAID METALSURFACE, ONE OF SAID REACTANTS BEING AN BASIC AMINE AND THE OTHER OFSAID REACTANTS BEING AN ACIDIC MATERIAL SELECTED FROM THE GROUPCONSISTING OG WEAK ACIDS AND ANHYDRIDES OF WEAK ACIDS, AT LEAST ONE OFSAID REACTANTS BEING IN THE VAPOR STATE AT THE TIME OF APPLICATION ANDBEING PRESENT IN THE GASEOUS MEDIUM AROUND SAID SURFACE IN ACONCENTRATION OF AT LEAST TEN PERCENT, SAID AMINE AND SAID ACIDICMATERIAL BEING BROUGHT SEPARATELY INTO CONTACT WITH SAID SURFACE FORSAID REATING THEREON.